Loop detector of a branching device for a multiplexed audio-video signal transmission system

ABSTRACT

A multiplexed audio-video signal transmission system has a camera-equipped intercom which produces a balanced audio-video signal. A twisted pair-wire connected to the intercom transmits a multiplexed signal of audio signal, video signal modulated to a frequency band not overlapping the audio signal frequency, and DC power. The audio signal and video signal are carried in the pair-wire in opposite phase in a balanced condition, and the DC power is carried between the pair-wire. A relay device is inserted in the pair-wire to refine the video signal. Main line branching device and door branching device are provided to branch out the multiplexed signal to a television equipped control unit 17.

This application is a division of application Ser. No. 08/227,341, filedApr. 14, 1994, now U.S. Pat. No. 5,778,303.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a multiplexed audio-video transmissionsystem through a pair-wire, such as used for connecting telephones, andusing a main line branching device and to a branch line branching deviceenabling multiplex transmission of video signals and audio signalsthrough the pair-wire, thereby enabling a point-to-multipoint connectionmodel. This type of system is typically used in security systems usingthe pair-wire to provide two-way audio and one-way video viewing.

2. Description of the Prior Art

FIG. 10 shows a prior art audio-video transmission system such asapplied in an audio-video security system.

The audio-video security system comprises a camera-equipped audioterminal 201 and a television-equipped audio terminal 217. Thecamera-equipped audio terminal 201, of which the exteriorcamera-equipped intercom unit, or interphone unit, is typical, transmitsthe multiplexed audio signal, DC current and video signal, which ismodulated to a frequency band not overlapping the audio signal frequencyband. The television-equipped audio terminal 217, of which the interiortelevision-equipped intercom unit is typical, comprises an audiotransmitter 215 for supplying DC current and receiving and transmittingthe audio signal, and a video receiver 216 for receiving the videosignal. The camera-equipped audio terminal 201 and television-equippedaudio terminal 217 are connected by means of a specific wire pair 202a,202b in the main line paired cable comprising plural wire pairs, aspecific wire pair 207a, 207b in the branch line paired cable comprisingplural wire pairs, the dedicated audio signal transmission cable 213a,213b, and the dedicated video signal transmission cable 214a, 214b (inFIG. 10, only one pair each of the main and branch line paired cables isshown).

The main line wire pair 202a, 202b and the branch line wire pair 207a,207b are connected by the main line branching device 205; the branchline wire pair 207a, 207b, audio signal transmission wires 213a, 213b,and video signal transmission wires 214a, 214b are connected by thebranch line branching device 210. A main line terminator 204 is providedat the end of the main line wire pair 202a, 202b for the impedancematching of the main line wire pair 202a, 202b in the video signalfrequency band and having a high impedance characteristic at frequencieslower than the video signal frequency band. A branch line terminator 208is provided at the end of branch line wire pair 207a, 207b for theimpedance matching of the branch line wire pair 207a, 207b in the videosignal frequency band and having a high impedance characteristic atfrequencies lower than the video signal frequency band.

The main line branching device 205 is connected at an appropriateposition in the main line wire pair 202a, 202b and in turn to the branchline wire pair 207a, 207b. When the audio signal is passed to the branchline wire pair 207a, 207b, DC current is input from terminals 203a,203b, the video signal is received balanced at a high input impedancefrom terminals 203a, 203b, and is output balanced to terminals 206a,206b at an output impedance matched with the branch line wire pair 207a,207b at the frequency band of the video signal.

The branch line branching device 210 is connected at an appropriateposition in the branch line wire pair 207a, 207b, one end of the audiosignal transmission wires 213a, 213b, and one end of the video signaltransmission wires 214a, 214b. DC current is supplied from the terminals211a and 211b. The audio signal is transmitted bidirectionally betweenterminals 209a, 209b and terminals 211a, 211b. The video signal isreceived at a high input impedance balanced from terminals 209a, 209b,and is output unbalanced to terminals 212a, 212b at an output impedancematched with the video signal transmission wires 214a, 214b at thefrequency band of the video signal.

The main line branching device 205 and branch line branching device 210of the prior art audio-video transmission system as above typically usea transformer.

FIG. 11 is a circuit diagram of a conventional transformer-type mainline branching device. This transformer-type main line branching device205 comprises a balanced transmitter 273 for transmitting the videosignal, and a low band signal mixer 274 for transmitting the audiosignal.

At the one balanced transmitter 273, the video signal input to terminals203a and 203b passes capacitors 276a, 276b, is applied at a high inputimpedance with respect to the characteristic impedance of the specificpair-wire, to terminals 203a and 203b, supplied to the input-side coilof the transformer 277, of which the output impedance characteristic ismatched to the characteristic impedance of the specific pair of wiresused in the branch line cable, and is thus output from the output-sidecoil of the transformer 277 through capacitors 278a, 278b to terminals206a and 206b. Note that passage of the audio signal and DC current isblocked at this time by capacitors 276a, 276b and capacitors 278a, 278b.

At the low band signal mixer 274, the audio signal is input throughcapacitors 280a, 280b and coils 281a, 281b, enabling bidirectionaltransmission between terminals 203a, 203b and terminals 206a and 206b.Coils 281a, 281b block passage of the video signal at this time, andcapacitors 280a, 280b block passage of the DC current.

FIG. 12 is a circuit diagram of a prior art transformer-type doorbranching device 210. This transformer-type door branching device 210comprises an unbalanced transmitter 284 for transmitting the videosignal and balanced-to-unbalanced conversion, and a low band signalseparator 283 for transmitting the audio signal.

At the unbalanced transmitter 284, the video signal input balanced toterminals 209a, 209b is applied to capacitors 289a, 289b and at a highinput impedance to terminals 212a, 212b, passes transformer 290, ofwhich the output impedance characteristic is matched to thecharacteristic impedance of the video signal transmission cable (ofwhich coaxial cable is typical), and is output unbalanced from thetransformer 290 to terminals 212a, 212b. The audio signal and DC currentare blocked by capacitors 289a, 289b at this time.

At the low band signal separator 283, the audio signal is input throughcoils 286a, 286b and capacitors 287a, 287b to enable bidirectionaltransmission between terminals 209a, 209b and terminals 211a, 211b. Thevideo signal is blocked by coils 286a, 286b, and the DC current isblocked by capacitors 287a, 287b at this time.

With the prior art transformer-type main line branching devices andbranch line branching devices, however, it is necessary to usetransformers 277 and 290 with a uniform high input impedance at thevideo signal frequency band, and manufacturing such transformers istechnically difficult. This makes it necessary to use transformers witha nonuniform high input impedance in the video signal frequency band,resulting in deterioration of video signal transmission quality.

The transmitted signal level also tends to drop when the video signal ispassed because the number of turns in the input-side coil of thetransformer is significantly greater than the number of turns in theoutput-side coil.

In addition, the high cost of the transformers necessarily increases thecost of the overall system, while practical limits to reducingtransformer size limit the size and weight reductions that can beachieved in the individual units.

SUMMARY OF THE INVENTION

Therefore, an object of the present invention is to provide atransformer-less multiplexed audio-video signal transmission system sothat a uniform high input impedance characteristic can be assured in thevideo signal frequency band; dropping of the signal transmission levelwhen the video signal is passed can be prevented; cost increases in theoverall system can be reduced; and unit size and weight can be reduced.

Another object of the present invention is to provide a transformer-lessmultiplexed audio-video signal transmission system for transmittingthrough a pair-wire a multiplexed signal of audio signal, video signalmodulated to a frequency band not overlapping the audio signalfrequency, and DC current, in which the audio signal and video signalare carried in the pair-wire in opposite phase in a balanced condition,and the DC current is carried between the pair-wire.

To achieve these and other objects, the multiplexed audio-video signaltransmission system according to the present invention comprises:

camera-equipped subunit means having a camera, a speaker and amicrophone for producing a first balanced audio-video signal;

first pair-wire means connected to the subunit means for transmittingthe first balanced audio-video signal;

a relay device connected to the first pair-wire means, the relay deviceseparating a balanced video signal from the first balanced audio-videosignal to refine the balanced video signal and combining the refinedbalanced video signal with a separated audio signal to produce a secondbalanced audio-video signal, the relay device comprising:

(i) a balanced-to-unbalanced convertor for converting the separatedbalanced video signal to unbalanced video signal;

(ii) an automatic gain control unit for amplifying the unbalanced videosignal; and

(iii) a balanced transmitter for producing an amplified balanced videosignal which is a refined version of the balanced video signal;

a second pair-wire means connected to the relay device for transmittingthe second balanced audio-video signal;

a line branching device connected to the second pair-wire means forreceiving and refining the second balanced audio-video signal and forproducing a third balanced audio-video signal;

a third pair-wire means connected to the branching device fortransmitting the third balanced audio-video signal;

a door branching device connected to the third pair-wire means forreceiving the third balanced audio-video signal and for separating theaudio signal and the video signal; and

a television-equipped control unit connected to the door branchingdevice for viewing an image captured by the camera using the videosignal and for effecting two-way conversation with the camera-equippedsubunit means using the audio signal.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more fully understood from thedetailed description given below and the accompanying diagrams wherein:

FIG. 1 is a circuit diagram of a multiplexed audio-video transmissionsystem through a pair-wire according to the preferred embodiment of theinvention,

FIG. 2a is a block diagram of a relay used in the circuit of FIG. 1,

FIG. 2b is a graph showing waveforms observed at various points in FIG.2a,

FIG. 3 is a circuit diagram of a balanced-to-unbalanced convertor showin FIG. 2a,

FIG. 4 is a circuit diagram of an automatic gain control device shown inFIG. 2a,

FIG. 5a is a circuit diagram of a main line branching devices shown inFIG. 2a,

FIG. 5b is a view similar to FIG. 5a, but showing a modificationthereof,

FIG. 6 is a block diagram of a door branching device shown in FIG. 1,

FIG. 7 is a circuit diagram of a balanced-to-unbalanced convertor shownin FIG. 6,

FIG. 8 is a circuit diagram of a loop detector shown in FIG. 6,

FIG. 9 is a circuit diagram of a video signal extractor shown in FIG. 6,

FIG. 10 is a block diagram of a multiplexed audio-video transmissionsystem according to a prior art,

FIG. 11 is a circuit diagram of a prior art main line branching deviceused in the system of FIG. 10, and having a transformer,

FIG. 12 is a circuit diagram of a prior art door branching device usedin the system of FIG. 10, and having a transformer.

DESCRIPTION OF PREFERRED EMBODIMENTS

The preferred embodiments of the present invention are described belowwith reference to the accompanying figures, of which FIG. 1 is a circuitdiagram showing a multiplexed audio-video signal transmission systemthrough two wires extending parallel and straight or twisted, such asused in a telephone connection. The two parallel wires is hereinreferred to as a pair-wire. The systems disclosed herein are representedby a camera and monitor-equipped intercom security system.

The multiplexed audio-video signal transmission system shown in FIG. 1comprises camera-equipped door terminals 1 and 2, a television-equippedintercom terminal 17, and a central processing unit (CPU) 3. Thecamera-equipped door terminal 1 has a camera la, a speaker 1b and amicrophone 1c. Similarly camera-equipped door terminal 2 has a camera2a, a speaker 2b and a microphone 2c.

The camera-equipped door terminals 1 and 2 transmit multiplexed signalof the audio signal, DC current, and video signal. The video signal ispreferably frequency modulated to a frequency band not overlapping theaudio signal frequency band. The camera-equipped door terminals 1 and 2may be provided at a front and back entrance doors, respectively, of abuilding. Each of the camera-equipped door terminals 1 and 2 further hasa keyboard (not shown) by which a room number signal is produced byoperating the keys to designate a particular room in the building.

The television-equipped intercom terminal 17 comprises an audiotransmitter 18 for transmitting and receiving the audio signal, and avideo receiver 19 for receiving the video signal and supplying DCcurrent to a television TV and also to branching device 13. The audiotransmitter 18 has a CPU which compares the received room number signalwith a particular room number signal allocated to that CPU. When thereceived room number signal matches with the allocated room numbersignal, that particular television-equipped intercom terminal 17 isactivated.

The center control device 3 has a CPU which provides bus control betweenthe camera-equipped door terminals 1, 2 and the television-equippedintercom terminal 17, which can be connected where required to thesecurity system, thus enabling audio and video signal transmission. Thecenter control device 3 is further connected to the one camera-equippeddoor terminal 1 via wire pair a1, a2, and to the other camera-equippeddoor terminal 2 via wire pair b1, b2,

The center control device 3 and television-equipped intercom terminalare connected by main lines c7 and d7, branch line e7, dedicated audiosignal transmission cable f7, and a dedicated video signal transmissioncable, which is a pair of co-axial cables g1 and g2. Main line c7comprises two pairs of data lines c1, c2 and c3, c4 and one pair ofcontrol lines c5 and c6. Main line d7 comprises two pairs of data linesd1, d2 and d3, d4 and one pair of control lines d5 and d6. Branch linee7 similarly comprises two pairs of data lines e1, e2 and e3, e4 and onepair of control lines e5 and e6. The dedicated audio signal transmissioncable f7 comprises two pairs of data lines f1, f2 and f3, f4 and onepair of control lines f5 and f6. The data lines, such as c1, c2, areprovided for sending data (multiplexed signal of the audio signal, DCcurrent, and video signal) on wire pair a1, a2 or wire pair b2, b2, asselected by the center control device 3. The control lines, such as c5,c6, are provided for sending control signal, such as a room numbersignal produced by the keyboard in the camera-equipped door terminal 1or 2.

The data transmitted through data lines, such as c1, c2, are completelyin opposite phase, such as shown at inputs S1 and S2 in FIG. 2a. Such asignal in opposite phase is referred to as a balanced signal.

The two pairs of data lines c1, c2 and c3, c4 and one pair of controllines c5 and c6 in main line c7, and the two pairs of data lines d1, d2and d3, d4 and one pair of control lines d5 and d6 in main line d7 areconnected through relay 4. The two pairs of data lines d1, d2 and d3, d4and one pair of control lines d5 and d6 in main line d7 are connectedthrough main line branching device 7 with the two pairs of data linese1, e2 and e3, e4 and one pair of control lines e5 and e6 in branch linee7.

The two pairs of data lines e1, e2 and e3, e4 in branch line e7 areconnected with the two pairs of data lines f1, f2 and f3, f4 in thededicated audio signal transmission cable f7 and with the dedicatedvideo signal transmission coaxial cables g1 and g2 through the doorbranching device 13. The control lines e5 and e6 in branch line e7 areconnected directly to the control lines f5 and f6 in the dedicated audiosignal transmission cable f7.

The ends of the two pairs of data lines d1, d2 and d3, d4 in main lined7 are connected to a main line terminator 10, which matches thecharacteristic impedance of the data lines d1, d2 and d3, d4 in thevideo signal frequency band and has a high impedance characteristic atfrequencies lower than the video signal frequency band so as to preventany echo reflection of the audio-video signal at the terminator 10. Forthe same purpose, the ends of the two pairs of data lines e1, e2 and e3,e4 in branch line e7 are connected to a branch line terminator 14, whichmatches the characteristic impedance of the data lines e1, e2 and e3, e4in the video signal frequency band and has a high impedancecharacteristic at frequencies lower than the video signal frequencyband.

The relay 4 is placed at any suitable relay position between main linec7 and main line d7 to enable bidirectional transmission of the audiosignal and DC current between the main lines c7 and d7; particularlybetween two pairs of data lines c1, c2 and c3, c4 in main line c7 andthe two pairs of data lines d1, d2 and d3, d4 in main line d7. DCcurrent is received from the two pairs of data lines c1, c2 and c3, c4in main line c7.

The relay 4 comprises relay devices 5 and 6 for receiving the balancedsignal. In each of the relay devices 5 and 6, the balanced signal isconverted to unbalanced signal, which is a signal between a ground leveland the video signal. The characteristic impedance of the two pairs ofdata lines c1, c2 and c3, c4 in main line c7 match with the inputimpedance of the respective relay devices 5 and 6. In each of the relaydevices 5 and 6, the video signal is amplified to the rated transmissionsignal level. The unbalanced is again converted to the balanced signalin each of the relay devices 5 and 6, and the balanced signal istransmitted to the two pairs of data lines d1, d2 and d3, d4 in mainline d7. Here again, the impedance matching is taken between the outputsof the devices 5 and 6 and the two pairs of data lines d1, d2 and d3, d4in main line d7 in the video signal frequency band. The control lines c5and c6 of main line c7 are directly connected with the control lines d5and d6 of main line d7.

The main line branching device 7 is placed between the one end of thebranch line e7 and any suitable position in main line d7 to enablebidirectional transmission of the audio signal between the two pairs ofdata lines d1, d2 or d3, d4 in main line d7 and the two pairs of datalines e1, e2 or e3, e4 in branch line e7. DC current is alsotransmitted.

The main line branching unit 7 comprises main line branching devices 8and 9 for receiving balanced signal of the video signal from the twopairs of data lines d1, d2 and d3, d4 in main line d7 at a high inputimpedance level, and for transmitting balanced signal along the twopairs of data lines e1, e2 and e3, e4 in branch line e7. Again, theimpedance matchings at the video signal frequency band are taken at theinput and output of the main line branching devices 8 and 9. The controllines d5 and d6 of main line d7 are directly connected to the controllines e5 and e6 of the branch line e7.

The door branching device 13 is connected at an appropriate position inthe branch line e7 between the end of the dedicated audio signaltransmission cable f7 and the ends of the dedicated video signaltransmission cables g1, g2, which are provided to a particular room. DCcurrent is supplied from the dedicated video signal transmission cablesg1, g2. The door branching device 13 enables bidirectional transmissionbetween the data lines e1-e4 in branch line e7 and the data lines f1-f4of the dedicated audio signal transmission cable f7; receives the videosignal balanced at a high input impedance level from the data linese1-e4 in branch line e7; detects a DC short-circuited state from the twodata line pairs f1, f2 and f3, f4 of the dedicated audio signaltransmission cable f7; and transmits unbalanced signal of the videosignal taken from one line pair that has been detected asshort-circuited, through the video signal transmission cables g1, g2.The impedance matching in the video signal frequency band is taken ateach of input and output of the door branching device 13. The controllines e5 and e6 of the branch line e7 are directly connected to thecontrol lines f5 and f6 of the dedicated audio signal transmission cablef7.

The preferred embodiment of the relay device 5 is described next withreference to FIGS. 1-4.

FIG. 2a is a block diagram of the relay device 5 provided in the relay4. The relay device 6 has the same structure as the relay device 5.Input terminals S1 and S2 are connected to the either one of the twopairs of data lines c1, c2 and c3, c4 in main line c7, and outputterminals S6 and S7 are connected to either one of the two pairs of datalines d1, d2 and d3, d4 in main line d7. The relay device 5 has abalanced-to-unbalanced convertor 5a, a band pass filter 5b, an automaticgain control device 5c, a balanced signal transmitter 5e, a low passfilter 5f, and a power unit 5d.

FIG. 2b shows waveforms of the signals at inputs and outputs of thecircuits 5a, 5b, 5c and 5e. In FIG. 2b, the waveforms in the left-handcolumn show a case when the input balanced signals S1 and S2 have arelatively high amplitude, and those in the right-hand column show acase when the input balanced signals S1 and S2 have a relatively lowamplitude.

The balanced-to-unbalanced convertor 5a takes impedance matching at thevideo signal frequency band at its input with the data lines c1, c2;presents a high input impedance in the audio signal and DC currentfrequency band so as to block the audio signal and DC current; receivesa balanced video signal (S1 and S2 in FIG. 2b), converts the balancedsignal to an unbalanced video signal (S3 in FIG. 2b), and transmits theunbalanced signal.

The automatic gain control device 5c amplifies the video signal output(S4 in FIG. 2b) produced from the band pass filter 5b to produce anunbalanced signal having an appropriate amplitude.

The balanced transmitter 5e converts the video signal obtained from theautomatic gain control device 5c from the unbalanced signal format tobalanced signal format; takes impedance matching in the video signalfrequency band with the data lines d1, d2 in main line d7; and presentsa high output impedance in the audio signal and DC current signalfrequency band while transmitting the video signal.

The low pass filter unit 5f has two coils with a high impedancecharacteristic in the video signal frequency band and a low impedancecharacteristic in the audio signal frequency band, so that audio signalcan be transmitted therethrough in bidirection.

The power unit 5d is provided for supplying DC current at a high inputimpedance in the video signal and audio signal frequency bands andconverts the input current to the predetermined voltage level to supplypower to the balanced-to-unbalanced convertor 5a, automatic gain controldevice 5c, and the balanced transmitter 5e.

Referring to FIG. 3, a detail of the balanced-to-unbalanced convertor 5ais shown. The balanced-to-unbalanced convertor 5a has a balancedreceiver 41 formed by a differential circuit and an unbalancedtransmitter 42.

The balanced receiver 41 comprises capacitors C1 and C2 and resistor R1,which are for the impedance matching with respect to the lines c1 andc2. The balanced receiver 41 further comprises transistors T1 and T2with capacitors C3, C4 and bias resistors R2, R3 and R4, R5 connected tothe bases of the transistors T1 and T2. Also, resistors R10 and R11 areconnected to the emitters of the transistors T1 and T2. The capacitorsC3, C4 have a high impedance with respect to the audio signal frequencyband. The bias resistors 35, 36 and 37, 38 are for matching thecharacteristic impedance of the input lines.

The unbalanced transmitter 42 comprises a transistor T3 with its emitterand base connected to the collectors of transistors T1 and T2,respectively. The unbalanced transmitter 42 further comprises resistorsR6, R7, R8 and R9 and a capacitor C13. The resistances of resistor R6 issmaller than is that of resistor R7 so that the base and emitter voltageof the transistor T3 is made approximately equal in terms of DC level.Resistor R8 and capacitor C13 are provided for the impedance matching.The transistor T3 operates such that the signals appearing at its baseand emitter are transmitted to the collector thereof when said signalshave opposite phase, but are cut off when said signals have the samephase. In this manner, the balanced video signal, which is a pair ofopposite phase signals, is converted to unbalanced video signal.

By the balanced-to-unbalanced convertor 5a of the embodiment thusconfigured, a uniform input impedance is held in the video signalfrequency band by means of the bias resistances R2-R5 provided in thebalanced receiver 41, and can thus avoid, during video signaltransmission, deterioration of transmission quality such as caused bythe phase distortion. In the case where a transformer is used in thebalanced-to-unbalanced convertor as in the prior art, the phasedistortion would occur often, and it is difficult to remove such a phasedistortion.

It is also possible to avoid deterioration of audio signal transmissionquality due to a loss of AC impedance because the capacitors C3 and C4can hold a high AC impedance level between terminals S1 and S2 in theaudio signal frequency band.

Referring to FIG. 4, a detail of the automatic gain control device 5c isshown. Terminal S4 is connected to the band pass filter 5b, terminal S5is connected to the balanced transmitter 5e. The automatic gain controldevice 5c has a half-wave rectification circuit 51, a differentialamplifier 52, and an amplification adjustment circuit 53.

The half-wave rectification circuit S1 comprises a transistor 55 withthe terminal S4 connected to the base, the emitter connected to anemitter resistor 56 connected in parallel with a capacitor 57, and thecollector connected to resistor 54. A diode 58 is provided with itscathode connected to the transistor 55 collector and the anode connectedto parallel-connected pull-up resistor 60 and capacitor 59. The videosignal input from the terminal S4 is half-wave rectified and smoothed bythe half-wave rectification circuit 51, obtaining a high DC voltageequivalent to the attenuated signal level of the video signal.

The differential amplifier 52 has a transistor 62 with an emitterresistor 61 and its base connected to the anode of the diode 58. Aconstant current supply 64 is connected to the collector of thetransistor 62 and one side of the pull-up resistor 63. A transistor 66is provided with an emitter resistor 65, a collector resistor 67, andits base connected to the collector of transistor 62. A DC voltagedifferentially amplified by the differential amplifier 52 is obtainedfrom DC voltage output by the half-wave rectification circuit 51, andthe current supplied to the pull-up resistor 63 is amplified an amountequivalent to the increase in the DC voltage input to the transistor 62to amplify the DC voltage of the collector resistor 67.

The amplification adjustment circuit 53 has a four-terminal FET 68 withgate G1 connected to terminal S4 through capacitor 70 and bias resistors71 and 72, and gate G2 connected to the collector of transistor 66 inthe differential amplifier 52. The drain of FET 68 is connected to adrain resistor 69 and also to terminal S5. The video signal input togate G1 of the FET 68 from the terminal S4 is amplified an amountequivalent to the DC voltage level obtained by the differentialamplifier 52 and input to gate G2, and is output from terminal S5.

The automatic gain control device 5c simplifies half-wave rectificationand smoothing operation particularly by the use of diode S8 and theparallel-connected pull-up resistor 60 and capacitor 59.

Setting the degree of amplification and the offset voltage adjustment isalso simplified by the use of the pull-up resistor 63 in the operatingcircuit, and adjusting the voltage input to gate G2 by adjusting theratio of the emitter resistance to the collector resistance of thetransistor 66.

Note that the relay 4 can be provided in plural as required in the mainline for transmitting the audio signal, video signal, and DC current.

Referring to FIG. 5a, the transformer-less line branching device 8comprises a balanced receiver 19, a balanced transmitter 20, a low bandsignal mixer 21, and a power supply unit 22. The line branching device 8receives the balanced signal and sends out a refined version of thebalanced signal.

The balanced receiver 19 receives, at a high input impedance, thebalanced video signal from one pair-wire, such as d1, d2, in the mainline d7. The balanced transmitter 20 matches the video signal impedanceto the characteristic impedance of the branch lines and transmits thesignals balanced. The low band signal mixer 21 is provided for thebidirectional transmission of the audio signal. The power supply unit 22converts the DC current supplied from the main line d1, d2 to a DCvoltage, and supplies this DC voltage to the balanced receiver 19 andbalanced transmitter 20.

The balanced receiver 19 comprises bias resistors 26a, 27a, 26b, and27b; capacitors 25a and 25b; parallel-connected source resistors 30a andamplification capacitor 31a; parallel-connected transistor 28a, to whichcollector resistor 29a is connected, emitter resistor 30b, andamplification capacitor 31b; and transistor 28b, to which collectorresistor 29b is connected. The base resistors 26a, is 27a, 26b, and 27bmaintain an impedance level sufficiently greater than the characteristicimpedance of the one wire pair d1, d2 in the main line in the videosignal frequency band, and the capacitors 25a and 25b pass the videosignal frequency band.

The balanced transmitter 20 comprises transistors 35a and 35b; capacitor32a and bias resistors 33a, 34a connected to the base of transistor 35a;capacitor 32b and bias resistors 33b, 34b connected to the base oftransistor 35b; matching resistor 37a and capacitor 38a connectedbetween the emitter of transistor 35a and terminal 39a; matchingresistor 37b and capacitor 38b connected between the emitter oftransistor 35b and terminal 39b; emitter resistor 36a connected to theemitter of transistor 35a; and emitter resistor 36b connected to theemitter of transistor 35b.

Matching resistor 37a matches the characteristic impedance of the branchline connected to terminals 39a, and capacitor 38a passes the videosignal. Matching resistor 37b matches the characteristic impedance ofthe branch line connected to terminal 39b, and capacitor 38b passes thevideo signal. The balanced video signals from terminals 39a and 39b arein complete opposite phase.

The balanced transmitter 20 matches the video signal output balanced bythe balanced receiver 19 to the characteristic impedance of the branchline connected to terminals 39a and 39b, and thus balance-transmissionof the video signal is enabled.

The low band signal mixer 21 comprises capacitor 40a and coil 41aconnected between terminals 23a and 39a, and capacitor 40b and coil 41bconnected between terminals 23b and 39b. Capacitors 40a and 40b pass theaudio signal, and coils 41a and 41b pass the audio signal while blockingthe video signal, thus enabling bidirectional transmission of the audiosignal between terminals 23a, 23b and terminals 39a, 39b, respectively.Note that the capacitors 40a and 40b and coils 41a and 41b are referredto as electronic components.

The operation of this transformer-less main line branching device 8 isdescribed next.

First, a uniform high input impedance is maintained at the video signalfrequency band by the bias resistors 26a, 27a, 26b, and 27b provided inthe balanced receiver 19, thereby making it possible to avoiddeterioration of transmission quality (of which phase distortion, whichis difficult to manage with a transformer, is typical) during videosignal transmission.

By using capacitors that are designed to maintain the level of thereceived video signal input to terminals 23a, 23b equal to the level ofthe transmitted video signal output from terminals 39a, 39b as theamplification capacitors 31a, 31b provided in the balanced receiver 19,it is also possible to avoid deterioration of the transmission signallevel caused by differences in the turns ratios of the input- andoutput-side transformer coils.

It is also possible to maintain a high AC impedance between terminals23a, 23b and terminals 39a, 39b, respectively, in the audio signalfrequency band by means of capacitors 25a, 25b and capacitors 38a, 38b,and thus avoid deterioration of audio signal transmission quality causedby reduced AC impedance.

It is to be noted that plural transformer-less main line branchingdevices 8 can be inserted at the desired locations to one wire pair inthe main line carrying the audio signal, video signal, and DC current.

Referring to FIG. 5b, a modification of the transformer-less linebranching device 8 is shown. In the balanced receiver 19, instead ofcapacitors 31a and 31b, a constant current source 31c can be used.

Referring to FIG. 6, a circuit diagram of a door branching device 13 isshown. Terminals 95, 96 and 97, 98 are connected to the two pairs ofdata lines e1, e2 and e3, e4 in branch line e7, terminals 99, 100 and101, 102 are connected to the two pairs of data lines f1, f2 and f3, f4in dedicated audio signal transmission cable f7, and terminals 103a and103b are connected to the dedicated video signal transmission cables g1,g2, respectively. Note that the dedicated video signal transmissioncables g1, g2 is coaxial cable.

The door branching device 75 comprises a balanced-to-unbalancedconvertor 77, a band pass filter 76, video signal extractor 79, loopdetector 78, rectifier 80, and power supply 81.

The balanced-to-unbalanced converting receiver 77 receives the videosignal at a high input impedance, and balanced-to-unbalanced convertsthe input signal. The balanced video signal applied to inputs 95 and 96are converted to unbalanced vide signal and produced from terminal 77a.Similarly, the balanced video signal applied to inputs 97 and 98 areconverted to unbalanced vide signal and produced from terminal 77b.

The band pass filter 76 is connected to capacitors 82, 83 and 84, 85 andcoils 86, 87 and 88, 89 in series, and passes the audio signals inputthrough terminals 95, 96 and 97, 98, to terminals 99, 100 and 101, 102,respectively.

The video signal extractor 79 extracts the video signal from one of thetwo video signals at inputs 179 and 180.

The loop detector 78 monitors the DC short-circuited state of terminals99 and 100 to control video signal output. When the terminals 99 and 100are open, the video signal received through terminals 97, 98 istransmitted through the video signal extractor 79, and when theterminals 99 and 100 are short-circuited, the video signal receivedthrough terminals 95, 96 is transmitted through the video signalextractor 79. In other words, the loop detector 78 is provided to detectwhich of the two camera-equipped subunits 1 and 2 is operated. Thesignals applied to terminals 99 and 100 indicate which of the twosubunits 1 and 2 is operated. Thus, the loop detector 78 producessignals which can select either one of the two input signals 179 and 180applied to the video signal extractor.

The resistor 91 provided in the rectifier 80 has an output impedancematched to the characteristic impedance of the dedicated video signaltransmission cables g1, g2 (FIG. 1) in the frequency band of the videosignal output from the video signal extractor 79. The rectifier 80comprises a pull-down resistor 90 and capacitor 92. The power supply 81comprises a coil 93 and capacitor 94, and supplies the DC voltage inputfrom the dedicated video signal transmission cables g1, g2 to thebalanced-to-unbalanced convertor 77, loop detector 78, and video signalextractor 79.

Referring to FIG. 7, a detail of the balanced-to-unbalanced convertor 77described above is shown. Terminals 95, 96 and 97, 98 are connected tothe respective bases of transistors 112, 115, 126, and 129 throughcapacitors 105, 106 and 119, 120, which have a low impedance in thevideo signal frequency band and a high impedance in the audio signalfrequency band, and high impedance bias resistors 107, 108 and 109, 110,and 121, 122 and 123, 124. Transistors 112 and 115 have their emittersconnected through a suitable resistor to a constant current source 113,and transistors 126 and 129 have their emitters connected through asuitable resistor to a constant current source 127. The collector oftransistor 112 is connected to resistor 111, and the collector oftransistor 126 is connected to resistor 125. The collector of transistor115 is connected to resistors 114 and also to capacitor 133 and resistor116. The collector of transistor 129 is connected to resistor 128 andalso to capacitor 134 and resistor 130. A transistor 117 is providedwhich has its base connected to the collector of transistor 115 and itscollector to a source resistor 118 and also to terminal 77a. Similarly,a transistor 131 is provided which has its base connected to thecollector of transistor 129 and its collector to a source resistor 132and also to terminal 77b.

The balanced video signals input to terminals 95 and 96 are converted tounbalanced video signal which is produced from terminal 77a. Similarly,the balanced video signals input to terminals 97 and 98 are converted tounbalanced video signal which is produced from terminal 77b.

By the balanced-to-unbalanced convertor 77, a uniform high inputimpedance is held in the video signal frequency band by the biasresistors 107, 108 and 109, 110, and 121, 122 and 123, 124, and can thusavoid deterioration of transmission quality (of which phase distortion,which is difficult to manage with a transformer, is typical) duringvideo signal transmission.

It is also possible to avoid deterioration of audio signal transmissionquality due to a loss of AC impedance because the capacitors 105, 106can hold a high AC impedance level between terminals 95 and 96, andcapacitors 119, 120 can hold a high AC impedance level between terminals97 and 98 in the audio signal frequency band.

Referring to FIG. 8, a detail of the loop detector 78 is shown. The loopdetector 78 comprises a transistor 151 having the base connected toterminal 158 through parallel-connected pull-up resistor 154a andcapacitor 155a and also through base resistor 156. Transistor 151 alsohas an emitter resistor 150. A transistor 152 is provided having thebase connected to terminal 159 through parallel-connected pull-downresistor 154b and capacitor 155b, and further having a base resistor157, and an emitter resistor 153.

A photocoupler 148 is provided of which the anode is connected to thecollector of transistor 151, and the collector of which is connected toterminal 142 through parallel-connected pull-up resistor 144 andcapacitor 145, and parallel-connected collector resistor 140a andcapacitor 141a. Another photocoupler 149 is provided of which thecathode is connected to the collector of transistor 152, the anode isconnected to the cathode of photocoupler 148, and the collector of whichis connected to terminal 143 through parallel-connected pull-downresistor 146 and capacitor 147, and parallel-connected collectorresistor 140b and capacitor 141b.

When the terminals 158 (connected to terminal 99) and 159 (connected toterminal 100) are DC open, the collector of photocoupler 148 is open tothe ground, and the emitter of the other photocoupler 149 isshort-circuited to the ground. When the terminals 158 and 159 are DCclosed, the collector of photocoupler 148 is short-circuited to theground, and the emitter of the other photocoupler 149 rises to the powersupply voltage level.

By the loop detector 78, a high impedance level can be held in the audiosignal and video signal frequency bands between terminals 99 and 100 bybase resistors 156 and 157.

It is also possible to avoid the effects of the audio signal and videosignal on the control signal output from terminals 142, 143 byeffectively short-circuiting the terminals in the audio signal and videosignal frequency bands by capacitors 155a and 155b.

Referring to FIG. 9, a detail of the video signal transmitter 79 isshown. The video signal transmitter 79 comprises a transistor 168 withthe base connected to terminal 179 through capacitor 161 and biasresistors 163, 164, and comprising a collector resistor 167 and emitterresistor 169; a transistor 171 with the base connected to terminal 180through capacitor 162 and bias resistors 165, 166, comprising acollector resistor 170 and emitter resistor 172, and havingamplification capacitor 175 and amplification resistor 176 connected tothe emitter; a switching transistor 177 with the base connected toterminal 181, the emitter connected to the collector of transistor 168,and the collector connected to terminal 183; and another switchingtransistor 178 with the base connected to terminal 182, the emitterconnected to the collector of transistor 171, and the collectorconnected to terminal 183.

When current flows from terminal 181, the video signal input to terminal179 is output to terminal 183, and when current flows from terminal 182,the video signal input to terminal 180 is output to terminal 183.

By the video signal transmitter 79, two video signals can be mixed foroutput to terminal 183 by controlling the voltage of terminals 181 and182 using the switching transistors 177 and 178.

As described hereinabove, by using the relay and door branching deviceof the invention in a multiplexed audio-video signal transmission systemthrough a pair-wire, of which the camera-equipped intercom securitysystem is typical, it is possible to provide a multiplexed audio-videosignal transmission system such that a uniform, high input impedancecharacteristic can be maintained in the video signal frequency band;dropping of the transmission signal level when the video signal ispassed can be prevented; the cost increase in the overall system can bereduced; the size and weight of the main unit can be reduced; receivedvideo signals can be selectively transmitted; and the video signalattenuated through long-distance transmission can be amplified to thespecified transmission signal level.

It is to be noted that while the relay of the invention has beendescribed as applied in the multiplexed audio-video signal transmissionsystem, of which the camera-equipped intercom security system istypical, the relay shall not be so limited and can be applied, forexample, as a line branching device.

Further note that when the relay of the invention is used as a linebranching device, it is not always necessary to provide an automaticgain control device for adjusting the input signal level to thepredetermined output signal level.

In addition, while the embodiments of the invention have been describedas applied in multiplexed audio-video signal transmission system throughpair-wire, of which the camera-equipped intercom security system istypical, the invention shall not be so limited, and can be applied inany paired cable transmission system, including, for example, in atwo-way audio transmission system such as a telephone system enablingtwo parties to converse.

Furthermore, while the embodiments of the invention have been describedas applied in multiplexed audio-video signal transmission system throughpair-wire, of which the camera-equipped intercom security system istypical, the invention shall not be so limited, and the transmissionsignals may be any single or plural types of signals.

In addition, while a photocoupler is used in the loop detector of theembodiments, the loop detector shall not be so limited, and any type ofoptical semiconducting device can be used.

Furthermore, while the circuits have been described using transistors,various other active electronic components, such as FETs, are alsopossible.

As will be known from the above descriptions of the preferredembodiments, a uniform, high input impedance characteristic can beassured in the frequency band of the input signal, and it is possible toprevent a drop in the transmission signal level when the input signal ispassed.

It is also possible to amplify input signals attenuated duringtransmission to the predetermined transmission signal level by adjustingthe degree of amplification.

It is also possible to select from among plural received input signalsfor transmission.

The present invention can also reduce the cost increase in the overallsystem, and can reduce the size and weight of the branching devices,relays, and thus the overall system configuration.

Although the present invention has been fully described in connectionwith the preferred embodiments thereof with reference to theaccompanying drawings, it is to be noted that various changes andmodifications are apparent to those skilled in the art. Such changes andmodifications are to be understood as included within the scope of thepresent invention as defined by the appended claims unless they departtherefrom.

What is claimed is:
 1. A loop detector connected to a receiving end of asystem for transmitting multiplexed signals of audio signals, videosignals modulated to a frequency band not overlapping an audio signalfrequency, and DC current, said audio signal and video signal beingcarried in a pair-wire in opposite phase in a balanced condition, and DCpower being carried between said pair-wire, said loop detectorcomprising:first and second inputs for receiving a DC current componentfrom said pair-wire, a receiving system having a high input impedance, ashort-circuit detector for detecting short-circuited state of the DCcurrent component; and an isolation system that isolates saidshort-circuit detector from said first and second inputs.
 2. A loopdetector according to claim 1, wherein said receiving system comprises:apull-up device having a low impedance in the video signal frequency bandand connected through a first impedance component to a first output; anda pull-down device having a low impedance in the video signal frequencyband and connected through a second impedance component to a secondoutput.
 3. A loop detector according to claim 2, wherein theshort-circuit detector produces a first level detection signal at afirst contact point between said first impedance a component and saidpull-up device, and produces a second level detection signal at a secondcontact point between the second impedance component and said pull-downdevice when a short-circuit occurs between said first and second inputs,and further produces the second level detection signal at said firstcontact point, and produces the first level detection signal at saidsecond contact point when an open-circuit occurs between said first andsecond inputs.
 4. The loop detector according to claim 1, said isolationsystem comprising a photocoupler.
 5. A loop detector connected to areceiving end of a system for transmitting multiplexed signals of audiosignals, video signals modulated to a frequency band not overlapping anaudio signal frequency, and DC current, said audio signal and videosignal being carried in a pair-wire in opposite phase in a balancedcondition, and DC power carried being between said pair-wire, said loopdetector comprising:first and second inputs for receiving a DC currentcomponent from said pair-wire; a receiving system having a high inputimpedance, said receiving system comprising a pull-up device having alow impedance in the video signal frequency band and connected through afirst impedance component to a first output, and a pull-down devicehaving a low impedance in the video signal frequency band and connectedthrough a second impedance component to a second output; and a shortcircuit detector for detecting a short circuited state of the DC currentcomponent.
 6. The loop detector according to claim 5, wherein said shortcircuit detector produces a first level detection signal at a firstcontact point between said first impedance component and said pull-updevice, and produces a second level detection signal at a second contactpoint between said second impedance component and said pull-down devicewhen a short circuit occurs between said first and second inputs, andfurther produces said second level detection signal at said firstcontact point, and produces said first level detection signal at saidsecond contact point when an open circuit occurs between said first andsecond inputs.